If you carry out activities as a self-employed person and simultaneously work for another person and these activities correspond to different Social Security Schemes, you must be registered in and pay contributions under both Schemes.
It firstly needs to be determined whether the worker's situation should fall within the Special Self-Employed Workers' Scheme. Article 2 of Decree 2530/1970, of 20th August, regulating the Special Regime for Self-employed Workers (Spanish acronym:RETA), defines a Self-employed individual as a person who habitually, personally and directly carries out economic activity for profit without such activity being conditioned by an employment contract. This also applies to the case of using the remunerated services of other persons. In any case, your corresponding Office of the Treasury General of the Social Security will be able to inform as to whether or not you should be included in the RETA.
With regard to the contribution, current Social Security regulations do not provide for any contribution discount, if the worker by virtue of their work activity needs to be simultaneously included in the General and Self-Employed Workers' Schemes.
Double contribution or multiple activity will provide the entitlement to the earning of benefits in each of the Schemes in which you are registered, if the relevant contributions are shown to be superimposed and so long as the requirements of each Scheme for such entitlement are met.